University of Virginia Physics Department Wavelength and Pitch - Musical Instruments A Physical Science Activity

Student Activity

Materials

• Straw
• Scissors
• Pipe of different lengths

Procedure

Flatten the end of a drinking straw, and cut the flattened end to a point to simulate a double reed such as an oboe. You would like the two tips to be almost touching each other. Place the "reed" end inside your mouth so your lips or tongue don't interfere with it, and blow. This takes a little practice. Cut straws to different lengths to get different pitches. If straws of different widths are available, place one inside the other to make a slide trombone. Try to cut holes in the straw and play it like a real instrument. Note how the pitch changes with length. Now hit a piece of pipe on the end with the palm of your hand. Notice the pitch of the sound. Do the same to other pieces of pipe of different lengths. How does the pitch correspond to length?

Explanation

When you blow on the end of the straw, the two pieces of the tip vibrate together. This makes a vibration, which is necessary to make sound. The tips vibrate at a specific frequency and wavelength that is determined by the length of the straw. The vibration travels down the straw and reflects from the end. This sets up a wave in the air in the straw; the vibration will bounce back and forth between the two ends. It is this vibration that you are hearing. Changing the length of the straw (by clipping it off, or by making a straw trombone) changes the wavelength and frequency, and so changes the pitch. Making a hole in the straw so it is like a real instrument lets the vibration bounce off from where the hole is, which will also change the pitch. Similarly, hitting the pipe causes it to vibrate, and the frequency and wavelength depend on the length.